This is still fully possible on Immutable distros (which is why the name is misleading, but unfortunately is what stuck- “image-based” is a better description) and uBlue has a mechanism for it- since they’re delivered using OCI containers, it’s trivial to fork or derive from the project and add, remove or tweak whatever you need. There’s also BlueBuild which is YAML but that’s a third party project.
There’s also an indev branch of Lidarr with real plugin support. I’ve been using it and https://github.com/allquiet-hub/Lidarr.Plugin.Slskd and it’s working well, although it doesn’t automatically search so it still requires some intervention. Does Soularr?